Date: Thu, 20 Feb 1997 01:11:32 -0800 From: Joe Perez Subject: Tire size, rear-ends, and horsepower. (sermon) > If you increase the tire size (ok, 205's arent much trouble...), you > increase the load on the engine. > > Which means it takes more pedal to get going, and more pedal to keep it > there up those hills, all though the trade off is it allows the crusing > RPM to be at a slightly higher speed. (exact number dont come to mind...) True, to a point. To move a given object at a given speed requires that a fixed amount of work be done. We measure this work in horsepower. The main reason for this is that in order for the car to be in a certain place, air cannot also occupy that space. We need to push it out of the way. The faster the object is moving, the more air (volume/time) needs to be displaced. Thus, more power is required. For example, to move a late-model stock car down Daytona International Speedway at 190 miles per hour requires something on the order of 700-800 horsepower. These cars aren't the most aerodynamic in the world, but 190mph is pretty respectable. That's escape velocity for a Bug. Likewise, to move a Beetle down the Interstate at 80 mph on level terrain on a calm day requires about 50-60 horsepower. No problem, you say, our engines are capable of that! Mabey, mabey not. The amount of horsepower produced by an engine at any given moment is directly proportional to the speed (RPM) at which the engine is turning. As the engine turns faster, it produces more and more power, until it's peak horsepower has been reached. According to my Great Book of Knowledge (an old 1971 Chilton's) The 1600cc Type 1 engine is rated at 60 SAE horsepower, but only at its peak of 4,400 rpm. At a lesser speed, it's making less power. I don't have a graph handy, but it's less. The slower you turn it, the less it makes. So here is the job of the transmission. By combining the primary drive gears (1'st through 4'th) and the final drive gear (rear end) we wish to provide the perfect ratio of engine RPM/road speed, such that the avaliable power at a given RPM is always greater than actual road speed at said RPM. For example, if it takes 30 horsepower to move at 45 miles per hour, and the engine produces 30 horsepower at 2,000 rpm (these are guesses, folks) we need to make sure that the ratio of the transmission is such that the engine is turning perhaps 2,500 rpm at 45 miles per hour. This allows us to maintain speed, with power in reserve. But there's been talk of tire size recently. The word Baja was mentioned, with it's visions of 30 inch tall tires. Talk of reducing engine RPM at speed, things like that. As you have guessed, installing larger rear tires will alter the gear ratio of the transmission, causing the engine to run at a lower RPM for any given road speed. It's exactly as if you were to install a different ratio rear-end. This is bad. Let's have a look back at the example given earlier. Say it takes 30 hp to move a bug 45 mph, and the engine makes 30 hp at 2,000 rpm. In stock form, we were turning 2,500 rpm, giving us plenty of power in reserve for accelerating and such. But now we install bigger rear tires. This alters the drive ratio. Let's say that now we are only turning 2,200 at 45 mph. We can still do it, but with less power to spare. Now, say we get out on the highway and start accelerating. Watch out for the semi, they don't even see you. ;) So we get our Bug up to 70 or so, and suddenly it wont go any faster! The pedal is floored, the engine is in perfect tune, it just isn't accelerating! And we're on I-75, where the speed limit IS 70, so this is rather embarassing. A Geo just passed us. What's wrong? We've run out of power. Thanks to our new tires, the motor is only turning 3,500 rpm. And we're making about 45 hp. We need more than this to keep accelerating, and we could make it, if our gearing were lower. But the engine is out of sync with the tranny, so to speak, and while the tranny is thinking "3,500 rpm, we must be going about 60. There isn't enough wind resistance to merit a higher rpm" the engine is crying out in pain "I'm maxed out here! I need to turn faster!!!" Again, these numbers are guesses, but they're close. So we're stuck at 70 mph, lacking the power to go any faster, because our engine can't turn fast enough. It's really quite enough to give a person fits. Of course, the opposite of this can also be true. Back home in Port Charlotte, I was friends with a couple who owned several bugs. Ric and Lisa. Nice people. Anyhow, one of their bugs was a very flashy-looking '71 Super, with a Pearlescant Purple paint job, velour interior, an 1835cc engine chromed all to the sky, and low-profile tires on 14" wheels. One day, they decided to aquire another Bug, this one a fairly stock '67. They drove up together, along Interstate 75, to get to wherever the car was. The minimum distance from Port Charlotte to anywhere is about 40 miles, and that's just to get a Krystal burger. On the return trip, they each drove their respective cars. Lisa in her cherry '71 and Rick in his newly aquired '67. During the trip, Ric was out in front, and noticed Lisa lagging constantly behind. When they got home, Ric was informed that while he was crusing along at a leasurely pace, Lisa was practically red-lining the engine in her car to keep up! The tires! Those darn low-profile tires, coupled with their early-model IRS transmission meant that while the '67 was easily within its element on the highway, the '71, although it had more than enough power, was improperly geared for highway use. Until now, it had been a city driver, with gobs of torque and horsepower. But on the highway, the cost of this street machine became apparent. What's a VW lover to do? Well, you can either install a bigger engine, one which produces adequate horsepower at a lower speed, or you can replace the final drive. Neither is the most fun task in the world, but swapping gears is a hell of a lot cheaper. For example, if your old final was a 3.88:1 (meaning for every turn of the axles, the transmission main pinion shaft turned 3.88 times) then perhaps a 4.125 (68-70) or even a 4.375 (autostick) gear set will get you back on track. Do the math. Speed in 4'th = RPM/(4'th gear ratio x final drive ratio) x 60 x ((tire circumference in inches / 12) / 5280) Tire circumference is roughly equal to tire diamater x 3.14 Unfortunately, unless you happen to be planning on a new transmission soon, this is mostly academic. Altering the final drive ratio requires complete disassembly of the transmission. The ring gear is easy to get out, just pull the differential. But the pinion carries the entire driving gear stack. So, since this is out of the question for most people, let this be a warning. Before you go altering your tire size, do be sure that your transmission will handle it. -=>Joe Perez<=- -=>pinhead@grove.ufl.edu<=-